Oolong tea is made from leaves of the same plant that green and black teas come from. The difference lies in how long the leaves ferment. Green tea leaves are unfermented, while leaves for black tea are fully fermented. Oolong comes from leaves that are partially fermented.
The most notable changes happen in a group of chemicals called catechins. These are strong antioxidants that may act directly on human cells. There is good evidence to support tea drinking leads to longevity.